Medusa’s eye brightens on October 31, 2021. | SkyNews
Medusa’s eye brightens on October 31, 2021 (Chris Vaughan, Starry Night Education)

This Week’s Sky: October 25 to 31, 2021

A third quarter Moon, greatest elongations for inner planets and rising Messier objects — check out what you can see in This Week’s Sky.

Monday, October 25 – Mercury at greatest western elongation (pre-dawn)

Mercury at greatest western elongation on October 25, 2021. | SkyNews
Mercury at greatest western elongation on October 25, 2021 (Chris Vaughan, Starry Night Education)

On Monday, October 25, the planet Mercury will reach a maximum angle of 18 degrees from the Sun, and peak visibility for the current morning apparition. Look for the innermost planet shining brightly, very low in the east-southeastern sky between about 6:15 and 7 a.m. in your local time zone. In a telescope (inset) Mercury will exhibit a 57-per-cent-illuminated waxing gibbous phase. Mercury’s position above the nearly upright morning ecliptic (green line) will make this an excellent apparition for Northern Hemisphere observers, but a poor one for those located near the Equator and farther south.

Monday, October 25 – Waning Moon near Messier 35 (all night)

Waning Moon Near Messier 35 on October 25, 2021. | SkyNews
Waning Moon Near Messier 35 on October 25, 2021 (Chris Vaughan, Starry Night Education)

When the waning gibbous Moon rises among the stars of Gemini after 9 p.m. local time on Monday, October 25, it will be positioned a finger’s width to the left (or 1 degree to the celestial northeast) of the large open star cluster named the Shoe-Buckle Cluster, or Messier 35. The two objects will share the view in binoculars (red circle) all night long, although the Moon will move farther from the cluster hour by hour. To better see the cluster, which is nearly as wide as the Moon, try hiding the Moon just outside the left-hand edge of your binoculars’ field of view.

Tuesday, October 26 – Io and Ganymede shadows cross Jupiter (from 08:08 to 10:22 UT)

Io and Ganymede shadows cross Jupiter on October 25, 2021. | SkyNews
Io and Ganymede shadows cross Jupiter on October 25, 2021 (Chris Vaughan, Starry Night Education)

Starting very early Tuesday, October 26, observers with telescopes in western North America and across the Pacific Ocean can watch the small black shadows of two of Jupiter’s moons cross the planet’s disk at the same time. At 1:08 a.m. Pacific Daylight Time (or 08:08 UT on October 26), Io’s small shadow will join Ganymede’s larger shadow while it is already crossing. The two shadows will appear together for 2.5 hours. Those with a clear view on Canada’s west coast will be able to see this celestial event very low on the horizon as Jupiter sets.

Thursday, October 28 – Third quarter Moon (at 20:05 GMT)

Third quarter Moon on October 28, 2021. | SkyNews
Third quarter Moon on October 28, 2021 (Chris Vaughan, Starry Night Education)

The Moon will officially reach its third quarter phase at 4:05 p.m. EDT, or 20:05 GMT, on Thursday, October 28. At third quarter our natural satellite always appears half-illuminated, on its western side toward the pre-dawn Sun. It rises in the middle of the night and remains visible in the southern sky all morning. The name for this phase reflects the fact that the Moon has completed three quarters of its orbit around Earth, measuring from the previous new Moon. The ensuing week of moonless evening skies will be ideal for observing deep-sky targets.

Friday, October 29 – Venus at greatest eastern elongation (after sunset)

Venus at greatest eastern elongation on October 29, 2021. | SkyNews
Venus at greatest eastern elongation on October 29, 2021 (Chris Vaughan, Starry Night Education)

On Friday, October 29, Venus will officially reach its widest separation of 47 degrees east of the Sun. Viewed in a telescope (inset), the planet will exhibit a half-illuminated phase. For best results, view the planet during evening twilight when contrast between the bright planet and the surrounding sky is reduced. After today, our sister planet will continue to brighten and increase in apparent disk diameter as it swings sunward ahead of inferior conjunction in early January.

Saturday, October 30 – The Andromeda Galaxy (all night)

The Andromeda Galaxy on October 30, 2021. | SkyNews
The Andromeda Galaxy on October 30, 2021 (Chris Vaughan, Starry Night Education)

In October, the Andromeda Galaxy is climbing the northeastern sky during evening. This large spiral galaxy, also designated Messier 31 and NGC 224, is 2.5 million light-years from us, and covers an area of sky measuring three degrees by one degree (or six by two full Moon diameters). Under dark skies, M31 can be seen with unaided eyes as a faint smudge located 1.4 fist diameters to the left (or 14 degrees to the celestial northeast) of Alpheratz, the star that forms the left-hand (northwestern) corner of the square of Pegasus. The three westernmost stars of Cassiopeia, Caph, Shedar and Navi (Gamma Cas), also conveniently form an arrow that points towards M31. Binoculars (red circle) will reveal the galaxy better. In a telescope, use low magnification and look for M31’s two smaller companion galaxies, the foreground Messier 32 and more distant Messier 110 (inset).

Sunday, October 31 – Medusa’s eye brightens (at 6:44 p.m. EDT)

Medusa’s eye brightens on October 31, 2021. | SkyNews
Medusa’s eye brightens on October 31, 2021 (Chris Vaughan, Starry Night Education)

Algol, also designated Beta Persei, marks the glowing eye of Medusa from Greek mythology, and is among the most accessible variable stars for skywatchers. During a ten-hour period that repeats every two days, 20 hours, and 49 minutes, Algol dims and re-brightens noticeably when a companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we perceive. Algol normally shines at magnitude 2.1, similar to the nearby star Almach in Andromeda. But when fully dimmed, Algol’s magnitude 3.4 is almost the same as Rho Persei (ρ Per), the star sitting just two finger widths to Algol’s lower right (or 2.25 degrees to the celestial south). On Sunday, October 31 at 6:44 p.m. EDT or 22:44 GMT, Algol will sit low in the northeastern sky and shine at its minimum brightness. Five hours later the star will return to full intensity from a perch nearly overhead in the eastern sky.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

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